1. Field of the Invention
The present invention relates to a wireless communication device and wireless communication method for executing communication priority control in a wireless LAN communication system, particularly with a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) method.
2. Related Background Art
In the CSMA/CA method, during a random period before a terminal performs transmission, virtual carrier sensing is performed, and verification is made as to whether another terminal is communicating with a base station, and then actual transmission of a packet is performed after waiting for the communication between the other terminal and the base station to end if the other terminal and the base station have been communicating. At this time all terminals have equal access to transmit. In virtual carrier sensing, after a channel becomes idle, only in an IFS (Inter Frame Space) period, a random number is generated within a provided CW (Contention Window) range, a random period is determined on the basis of this random number, and backoff control to be described below is performed in this random period. In backoff control, the calculated random number is set as an initial value, and control is performed to actually transmit a packet when the initial value reaches zero after reducing the initial value as time goes by. Here, IFS is prescribed by the wireless LAN standard IEEE802.11, and is a predetermined period in which idle detection is performed before transmission, and CW is the maximum value of the random number that can be obtained for the backoff, and is a necessary parameter for realizing user multiplexing. In IEEE802.11, CWmin (the minimum value of CW) and CWmax (the maximum value of CW) are prescribed. The value of the CWmin is used to calculate the random number for a first transmission, and backoff is performed for a repeated transmission with a CW that doubles the CWmin. Note that the CWmax is the upper limit of the CW. Communication in which a plurality of terminals use the same channel in common is made possible with this backoff control having a random quality. However, in this method, it is possible for a plurality of terminals to transmit a packet at the same time, and in this case a collision of packets will occur, leading to packet receiving error and lowering of transmission quality. Particularly, the lowering of quality is remarkable in real-time type applications such as VoIP.
As prior art relating to priority control in communication methods such as this, there is EDCA (Enhanced Distributed Channel Access) prescribed by IEEE802.11e as shown in the document “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (P Y) specifications: Amendment 7: Medium Access Control (MAC) Quality of Service (QoS) Enhancements”. In EDCA, there are four types of priority for packets, and packets with a high priority are assigned waiting times for transmission, such as IFS, CWmin, and CWmax, that are short to give these packets priority to be transmitted. In this manner, relative priority control is possible in a communication environment in which various applications of voice, data, and the like coexist.